For example, in a communication network of Ethernet, a ring type topology is formed by switches as a plurality of relay devices connected in a ring shape. Incidentally, “Ethernet” is a registered trademark. In the ring type topology, a plurality of switches is connected in a ring shape by two ports of each switch connected to ports of the other switches. The switches form two redundant transfer paths therebetween.
JP 2013-192128 A discloses to mirror (that is, copy) a frame by a port mirroring function of one of a plurality of switches providing a network, and to transmit the mirrored frame to a measuring device connected to another switch.